(a) Medical Conditions Caused by Abnormal Tissue Water Content:
A variety of different medical conditions are associated with abnormal tissue water content. Examples of such medical conditions include, but are not limited to: edema (including brain edema), ischemia, internal bleeding (including intraperitoneal bleeding), dehydration, and extravasation.
A change in water content occurring in an organ or a tissue sample over time can be very indicative of a medical condition developing. As will be explained below, different systems have been developed to assess tissue water content properties (and changes in such conditions over time). Such systems can be particularly useful in diagnosing the onset of various medical conditions. However, as will be shown, existing systems all suffer from various disadvantages.
(b) Edema and Ischemia:
Tissue edema is a pathological condition involving an increase in the amount of fluid in tissue. The accumulation of fluid can be extracellular, intracellular or both. Extracellular edema is caused either by increased ultrafiltration or decrease in re-absorption. Intracellular edema can be caused by ischemia and the resulting intracellular hyperosmolarity or as a consequence of extracellular hypotonicity. Independent of the edema type, the condition is one in which the amount of liquid in the tissue increases and the balance is changed, usually as a function of time after an event has occurred. Tissue edema is of substantial concern when it occurs in the brain or in the lung. In the brain, extracellular edema develops in a delayed fashion, over a period of hours or days, after a large hemispheric stroke and is a cause of substantial mortality. Ischemic brain edema begins with an increase in tissue Na+ and water-content and continues with blood brain barrier breakdown and infarction of both the parenchyma and the vasculature itself.
A study of the Center for Disease Control and Prevention for the period from 1995 to 2001 indicates that at least 1.4 million annual traumatic brain injuries occur in the USA alone. These resulted in about 1.1 million emergency department visits, 235,000 hospitalizations and about 50,000 deaths. About 1,100 incidents per 100,000 in population occur in the age group from 0 to 4 years. Head injury causes more deaths and disability than any other neurological condition under the age of 50 and occurs in more than 70% of accidents. It is the leading cause of death in males under 35 yr old. Fatalities may not result from the immediate injury; rather, progressive damage to brain tissue develops over time. In response to trauma, changes occur in the brain that requires monitoring to prevent further damage.
Brain swelling can be caused by an increase in the amount of blood to the brain. Brain edema is one of the most important factors leading to morbidity and mortality in brain tumors. Cerebral edema, which is an increase in brain volume caused by an absolute increase in tissue water content, ensues. The accumulation of fluid can be extracellular, intracellular or both. Vasogenic edema results from trans-vascular leakage often caused by the mechanical failure of the tight endothelial junction of the blood-brain barrier and increased ultrafiltration or decrease in re-absorption. Vasogenic edema also results from extravasation of protein rich filtrate in interstitial space and accumulation of extracellular fluid. Cytotoxic edema is characterized by cell swelling. Cytotoxic edema is an intracellular process resulting from membrane ionic pump failure. It is very common after head injury and it is often associated with post-traumatic ischemia and tissue hypoxia. The primary mechanism is reduction of sodium-potassium ATPase pump efficiency due to local hypoxia and ischemia. This type of edema occurs in cancer with compression of microcirculation. Interstitial or hydrocephalic edema occurs when there is an accumulation of extracellular fluid in the setting of hydrocephalus. Intraventricular tumors or tumors that constrict ventricles can cause this type of edema.
Independent of the edema type, the condition is one in which the amount of liquid in the tissue increases or the balance is changed. Edema is of substantial concern when it occurs in the brain. The characteristics of brain edema, is that it develops in a delayed fashion, over a period of hours or days, after the brain trauma has occurred and is a cause of substantial mortality. Detection and continuous monitoring of edema in the brain is essential for assessment of medical condition and treatment.
Pulmonary edema is often associated with lung injury and also requires continuous monitoring and treatment. Detection and continuous monitoring of edema in the brain and lung is useful for assessment of medical condition and treatment.
Ischemia of tissues and organs is caused by a change in normative physiological conditions such as deprivation of oxygen and blood flow. It can occur inside the body, for instance as a consequence of impediments in blood flow. Ischemia also can occur outside the body when organs preserved for transplantation are transported. Ischemia results in changes in the intracellular composition which is accommodated by changes in the water content properties of the intracellular and extracellular space and leads to cell death.
Therefore, in medical applications it is important to be able to detect changes in water content properties which are indicative of the occurrence of edema and ischemia.
(c) Internal and Interperitoneal Bleeding:
Trauma is the third most common cause of death in all age groups and the leading cause of death in the first three decades of life. Of all traumatic injuries abdominal and pelvic injuries contribute to about 20% of the fatalities. In addition, death from abdominal hemorrhage is a common cause of preventable death in trauma patients. Bleeding is the cause of one in four maternal deaths worldwide. Death may occur in less than two hours after the onset of bleeding associated with childbirth. In addition to trauma, abdominal bleeding also occurs in several post-surgery conditions. Unfortunately, early intraperitoneal bleeding cannot be detected by vital signs (rate pulse or blood pressure) and it becomes evident only after a critical amount of blood has found its way into the abdominal cavity. Therefore, death from abdominal hemorrhage is a common cause of preventable death in trauma patients. However, early detection of intraperitoneal bleeding may play a critical role in the patient survival.
(d) Extravasation:
Extravasation is the unwanted passage or escape of blood, serum, lymph or therapeutic drugs directly into body tissues. Signs and symptoms may include the sudden onset of localized pain at an injection site, sudden redness or extreme pallor at an injection site, or loss of blood return in an intravenous needle. Extravasation can lead to skin and tissue necrosis, and “Compartment Syndrome” (a pathologic condition caused by the progressive development of arterial compression and reduction of blood supply).
Similar to the medical conditions described above, extravasation results in a change in water content properties in the tissue (typically at or near an injection site). Thus, it would be desirable to detect extravasation, (preferably by a on-contact system). Unfortunately, no such system currently exists.
(e) Existing Systems for Assessing Tissue Water Content Properties—and Their Limitations:
Accumulation of fluid in tissue changes the electrical impedance of the tissue. This has suggested the use of bioelectrical impedance measurements to detect water content in the body since 1962, Edema and ischemia can be also detected with bioelectric measurements. With edema or ischemia, the ratio between extracellular and intracellular water changes. Since this should cause a shift in the beta dispersion frequency, bioimpedance spectroscopy based on measuring the changes in the overall impedance has been viewed as a likely way to produce information on edema and ischemia.
Another important method to evaluate and monitor edema is Electrical Impedance Tomography (EIT). EIT uses an array of electrodes (placed on the patient) to inject subsensory currents and measure the resultant voltages. The data is used to reconstruct a map of the electrical impedance of tissue. Unfortunately, a problem with electrical impedance tomography (EIT) is that it requires the placement of needles in contact with the tissue. Furthermore, EIT produces an image of the area showing the location of the change in water content properties. This is a time consuming process. In addition the details produced by imaging may not be needed in many applications of detection of water content properties.
Another way to detect edema and ischemia is by performing induction tomography. In this approach, induction currents rather than injection currents are used to produce a map of the electrical properties of tissue. The problem with this method is that the induction coils need to be large, i.e. are much larger than electrodes, and there are difficulties with using large number of coils for good imaging resolution. Furthermore the imaging outcome has the same overall attributes as EIT in regards to detection of edema and ischemia. In general, imaging and tomography are expensive and require many measurements. In the past, direct impedance measurements of ischemia have been used to assess the condition of organs preserved for transplantation. However as with EIT, this requires the placement of needles on the organ or tissue. This is cumbersome with organs and impossible in such tissues as the brain and the lungs or large volumes of the abdomen.
Tumor associated edema is visible on both CT and MRI. Unfortunately, the diagnostic is complicated by the fact that on CT it produces low signal, which can be confused with low-signal producing tumors. On MRI, the edematous brain produces a hypersignal, which may be confused with hypersignal producing tumors.
With regard to diagnosing abdominal injuries, there are two methods for rapid detection of intraperitoneal bleeding, FAST (focused assessment with sonography for trauma) ultrasound and peritoneal lavage. Evaluations of the practice in the evaluation of internal injuries show that physicians prefer to use FAST ultrasound over diagnostic peritoneal lavage (DPL) because DPL is invasive and most doctors have limited experience in DPL and interpreting the results. However, rural hospitals rarely have advanced imaging modalities such as CT scan or emergency ultrasound. As a result, emergency physicians in such centers are forced to rely on clinical examination and plain radiography alone. The lack of advanced imaging may delay the identification of patients who require transfer, or lead to inappropriate transfer of patients who are later found not to require trauma centre intervention.
In addition, most of the current bioelectronic techniques for detection of abdominal bleedings try to produce an image or information that will determine the site of bleeding. However, currently in the medical emergency departments initial evaluation and treatment is not geared towards identification of a specific abdominal injury, but rather to determine if one exists.